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Controlling Molecular Orientation in Solid Films via Self-Organization in the Liquid-Crystalline Phase Isaac K. Iverson, Sean M. Casey, Wonewoo Seo, and Suk-Wah Tam-Chang* Department of Chemistry, University of Nevada, Reno, Nevada 89557
Bradford A. Pindzola Department of Chemistry, University of California, Berkeley, California 94720-1460 Received September 28, 2001. In Final Form: January 25, 2002 We report the control of molecular orientation in solid films through self-organization and inducedorientation processes. We synthesized water-soluble cationic 3,4,9,10-perylene diimide (1) and studied its self-organization in aqueous solution. By UV-vis spectroscopy, H-aggregates of 1 were observed forming in solutions with concentrations as low as 10-7 M. At concentrations above approximately 0.1 M (7% w/w), these solutions were observed with polarized microscopy to form a chromonic N phase (a nematic lyotropic liquid crystalline phase) at room temperature. Upon induced alignment (by shearing) of the chromonic N phase on a glass substrate and removal of solvent, anisotropic solid films of the dichroic dye were produced. These films have dichroic ratio values that routinely exceed 25 and in some cases 30, making them excellent polarizers over the blue and green region. By use of a combination of polarized UV-vis and FT-IR spectroscopies, the orientation of the average molecular plane in these films was determined to be perpendicular to both the shearing direction and the substrate plane. Small-angle X-ray diffraction studies indicate that the molecules in the solid film possess a high degree of order.
Introduction The construction of exceedingly sophisticated molecular edifices capable of being manipulated into devices and materials is fast becoming one of the premier frontiers in chemistry.1 Alone, however, stepwise classical synthesis via covalent bonding can hardly be expected to produce mammoth supramolecular objects and devices. Inspired by natural molecular systems that have similar attributes, there is more and more effort being devoted to the study of molecular self-assembly and self-organization and to the design of man-made complex systems.2 Significant progress is being achieved in the control of self-assembly processes, and in the fashioning of solution-phase discrete molecular nanostructures.3 This strategy can also be used to generate large nondiscrete arrays of molecules. The fabrication of highly ordered self-assembled monolayer4 and multilayer5 thin films has proven to be highly successful. However, the manipulation of molecular order and macroscopic properties of bulk solids has proven to be a major challenge6 (aside from crystallizationswhich is itself a self-organization mechanism). To this end, we have sought to utilize the selforganization inherent in chromonic liquid crystals derived (1) (a) Moore, J. S. Curr. Opin. Solid State Mater. Sci. 1996, 1, 777788. (b) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89-112. (2) (a) Fyfe, M. C.; Stoddart, J. F. Acc. Chem. Res. 1997, 30, 393-401. (b) Rebek, J. Chem. Soc. Rev. 1996, 255-264. (c) Reichert, A.; Ringsdorf, H.; Schuhmacher, P. In Comprehensive Supramolecular Chemistry; Sauvage, J.-P., Hosseini, M. W., Eds.; Elsevier Science Ltd.: New York, 1996; Vol. 9, pp 313-350. (d) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 734-5. (e) Orr, G. W.; Barbour, L. J.; Atwood, J. L. Science (Washington, D.C.) 1999, 285, 1049-1052. (f) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421-432. (g) Fernandez-Lopez, S.; Kim, H.-S.; Choi, E. C.; Delgado, M.; Granja, J. R.; Khasanov, A.; Kraehenbuehl, K.; Long, G.; Weinberger, D. D.; Wilcoxen, K. M.; Ghadiri, M. R. Nature 2001, 412, 452-455. (h) Boal, A. K.; Ilhan. F.; Derouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746-748. (3) (a) Raymo, F. M.; Stoddart, J. F. Chem. Rev. 1999, 99, 16431663. (b) Vollmer, M. S.; Clark, T. D.; Steinem, C.; Ghadiri, M. R. Angew. Chem., Int. Ed. Engl. 1999, 38, 1598-1601.
from molecules containing a conjugated aromatic core with hydrophilic groups around the periphery that allow for solubility in water.7,8 These molecules often have a plank shape and aggregate in water as a result of π-stacking (4) (a) Bain, C. D.; Whitesides, G. M. Science (Washington, D.C.) 1988, 240, 62-63. (b) Gupta, V. K.; Skaife, J. J.; Dubrovsky, T. B.; Abbott, N. L. Science (Washington D.C.) 1998, 279, 2077-2080. (c) Houseman, B. T.; Mrksich, M. Angew. Chem., Int. Ed. Engl. 1999, 38, 782-785. (d) Sabapathy, R. C.; Crooks, R. M. Langmuir 2000, 16, 177782. (e) Xu, S.; Liu, G. Langmuir 1997, 13, 127-129. (f) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (g) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319-5327. (h) Shon, Y.-S.; Lee, S.; Colorado, R.; Perry, S. S., Jr.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556-7563. (5) (a) Mallouk, T. E.; Gavin, J. A. Acc. Chem. Res. 1998, 31, 209217. (b) Clark, S. L.; Hammond, P. T. Adv. Mater. 1998, 10, 1515. (c) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206-10214. (d) Malik, A.; Lin, W.; Durbin, M. K.; Marks, T. J.; Dutta, P. J. Chem. Phys. 1997, 107, 645-652. (6) (a) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science (Washington D.C.) 1997, 276, 384389. (b) MacDonald, J. C.; Whitesides, G. M. Chem. Rev. 1994, 94, 23832420. (c) Melendez, R. E.; Hamilton, A. D. Top. Curr. Chem. 1998, 198, 97-129. (d) MacGillivray, L. R.; Atwood, J. L. J. Am. Chem. Soc. 1997, 119, 6931. (e) MacGillivray, L. R.; Atwood, J. L. J. Solid State Chem. 2000, 152, 199. (7) (a) Lydon, J. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2B, pp 981-1007. (b) Attwood, T. K.; Lydon, J. E.; Hall, C.; Tiddy, G. J. T. Liq. Cryst. 1990, 7, 657-668. (c) Attwood, T. K.; Lydon, J. E.; Jones, F. Liq. Cryst. 1986, 1, 499-507. (d) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. Faraday Discuss. 1996, 104, 139-154. (e) Lydon, J. Curr. Opin. Colloid Interface Sci. 1998, 3, 458-466. (8) For other types of liquid crystals and examples of their applications, see: (a) Collings, P. J.; Patel, J. S. Handbook of Liquid Crystal Research; Oxford University Press: New York, 1997. (b) Collings, P. J. Liquid Crystals; Princeton University Press: Princeton, NJ, 1990. (c) Miller, S. A.; Kim, E.; Gray, D. H.; Gin, D. L. Angew. Chem., Int. Ed. Engl. 1999, 38, 3022-3026. (d) Sentman, A. C.; Gin, D. L. Adv. Mater. 2001, 13, 1398-1401. (e) Trzaska, S. T.; Swager, T. M. Chem. Mater. 1998, 10, 438-443. (f) Shah, R. R.; Abbott, N. L. Science (Washington, D.C.) 2001, 293, 1296-1299. (g) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Ko¨rblova, E.; Walba, D. M. Science (Washington, D.C.) 1997, 278, 1924-1927. (h) Walba, D. M.; Dyer, D. J.; Sierra, T.; Cobben, P. L.; Shao, R.; Clark, N. A. J. Am. Chem. Soc. 1996, 118, 1211-1212. (i) Schneider, F.; Kneppe H. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 1, pp 454-476.
10.1021/la011499t CCC: $22.00 © 2002 American Chemical Society Published on Web 03/28/2002
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Figure 1. Controlling molecular orientation in the solid phase via self-organization and induced-orientation processes.
interactions and entropic factors, generating liquidcrystalline mesophases in which the molecules possess liquidlike mobility and yet have a degree of long-range crystal-like orientational order. Although chromonic liquid crystals appear to have been observed at least 80 years ago, only recently have they gained wide acceptance as a distinct class of mesogens (liquid-crystalline-prone molecules)7 and their potential applications remain relatively little explored. We focused on studying water-soluble perylenebisdicarboximides (e.g., 1) as dichroic chromonic mesogens. Molecules with the perylenebisdicarboximide core have been well studied.9 These compounds show strong absorption of visible light and intense fluorescence emission. In addition, they exhibit photoconducting properties.10 Elegant procedures have been developed for the synthesis of nonionic perylenebisdicarboximides with side chains to increase their solubility in organic solvents or to bestow thermotropic liquid-crystalline behavior.11 These compounds have applications in sensors, photoconducting materials, and electroluminescent displays.9,10 In most applications, the molecules are randomly oriented and their intrinsic dichroic properties have not been fully exploited. Perylenebisdicarboximides with ionic groups12 are also known; however, their chromonic liquid-crystalline properties and potential applications have received little attention.
(9) (a) Law, K. Y. Chem. Rev. 1993, 93, 449-486. (b) Wu¨rthner, F.; Sautter, A.; Schmid, D.; Weber, P. J. A. Chem. Eur. J. 2001, 7, 894902. (c) Wu¨rthner, F.; Sautter, A. Chem. Commun. 2000, 445-446. (d) Daffy, L. M.; de Silva, A. P.; Gunaratne, H. Q. N.; Huber, C.; Lynch, P. L. M.; Werner, T.; Wolfbeis, O. S. Chem.-Eur. J. 1998, 4, 1810-1815. (e) Langhals, H. Heterocycles 1995, 40, 447-500. (f) Holtrup, F. O.; Mu¨ller, G. R. J.; Quante, H.; De Feyter, S.; De Schryver, F. C.; Mu¨llen, K. Chem.-Eur. J. 1997, 3, 219-225. (g) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L.; Wasielewski, M. R. Science (Washington D.C.) 1992, 257, 63-65. (10) (a) Gregg, B. A. J. Phys. Chem. 1996, 100, 852-859. (b) SchmidtMende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science (Washington, D.C.) 2001, 293, 1119-1122. (11) (a) Langhals, H.; Ismael, R.; Yu¨ru¨k, O. Tetrahedron, 2000, 56, 5435-5441. (b) Wu¨rthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem. Eur. J. 2001, 7, 2245-2253. (c) Cormier, R. A.; Gregg, B. A. J. Phys. Chem. B. 1997, 101, 11004-11006. (d) Cormier, R. A.; Gregg, B. A. Chem. Mater. 1998, 10, 1309-1319. (12) (a) Liu, Z.-R.; Rill, R. L. Anal. Biochem. 1996, 236, 139-145. (b) Tuntiwechapikul, W.; Lee, J. T.; Salazar, M. J. Am. Chem. Soc. 2001, 123, 5606-5607.
We have previously reported the formation of a chromonic liquid-crystalline phase by cationic perylene diimide 1 in aqueous solution and the unprecedented control of molecular orientation of 1 in solid films.13 We demonstrated that the self-organization of 1 in a liquid-crystalline phase allows for its induced bulk alignment on glass substrates via application of an external mechanical force such as shearing. Upon removal of the water by evaporation under ambient conditions, the macroscopic alignment of the liquid crystal, and hence the molecules, is transferred to the solid state, yielding an anisotropically ordered solid film that linearly polarizes light (Figure 1).13 In principle, this approach to manipulating molecular orientation in, and macroscopic properties of, solid films should be applicable to other chromonic mesogens. Further understanding the self-organizing properties of these liquid-crystalline compounds is important for the design of novel chromonic mesogens and for ultimately controlling the molecular order and macroscopic properties of their solid films. In this paper, we describe in detail the selforganization of 1 as a novel mesogen in aqueous solution, the study of the optical properties of the resulting anisotropically ordered films, and the determination of the average molecular plane orientation in these solid films. Experimental Section Compound 1 was synthesized by a modification to the literature procedure14 as reported previously and characterized by 1H and 13C NMR spectroscopy and high-resolution mass spectrometry. 13 The dilute aqueous solutions used in the UV-vis studies were prepared by dissolving the neutral amine of 1 in water and a slight molar excess of hydrochloric acid (HCl). This solution was then dried under vacuum. The dried salt 1 was then redissolved in the appropriate amount of water to produce the desired concentration. Doubly distilled water (Barnstead Mega-pure system MP-3A) was used in preparing the solutions. Concentrated aqueous solutions of 1 that exhibit liquid-crystalline phases were also made by dissolving the neutral amine in a stoichiometric amount of HCl and then shaking in a shaker/hot bath at 40 °C for at least 6 h. The solutions were then examined under a microscope to screen for particles and ensure the complete dissolution of 1. Sheared solid films of 1 were prepared by depositing the liquidcrystalline (LC) solutions of 1 on glass slides using a custom coating device (Figure 2). Wet film applicator rods (3/8 in. diameter and #3 or #5 wire size, Paul N. Gardner Co., Inc., Pompano Beach, FL) were pulled across the slide using 200 g “down weight” and 700 g “pulling weight” to ensure reproducible shearing forces and rod velocities. The UV-vis spectral data were collected on films formed using the #3 rod. Polarizing films prepared using (13) Iverson, I. K.; Tam-Chang, S.-W. J. Am. Chem. Soc. 1999, 121, 5801-5802. (14) Khromov-Borisov, N. V.; Indenbom, M. L.; Danilov, A. F. Khim. Farm. Zh. 1980, 14, 15-20.
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Figure 2. Drawing illustrating the function of the custombuilt coating apparatus. The rod is pulled without rolling across the glass substrate. the #3 or #5 rods showed similar colors and optical texture when observed under a polarizing microscope. Only the optical micrographs of the films prepared by the #5 rod are shown in this paper. Although these conditions were not optimized for the highest quality film production, films prepared with this coating device had more reproducible film quality than the films prepared by methods described previously.13 All substrates for these thin films were standard glass slides that had been treated with “piranha cleaning solution” (concentrated H2SO4/30% H2O2, 7:3 v/v) at 60 °C for 30 min, rinsed with copious amounts of distilled water followed by ethanol, and dried under vacuum. Caution: Piranha solution should be handled with great care and should not be allowed to contact significant amounts of oxidizable organic compounds. In some circumstances, it has detonated unexpectedly. All electronic spectra were obtained with a dual-beam Cary14 spectrophotometer fitted with an analog-to-digital (A/D) converter and software by OLIS, Inc. Visible spectra of aqueous solutions were taken in cuvettes of the desired path length. For path lengths of less than 1 mm, two 25 mm diameter quartz windows were pressed together on Teflon spacers of known thicknesses. All visible spectra of solid films were collected with the incident radiation normal to the surface of the substrate. The incident light was polarized with an 8-mm clear aperture Glan-Thompson Polarizer (Meadowlark Optics, Frederick, CO). A custom-built sample holder equipped with a rotation stage (Newport Corp., Irvine, CA) to which the glass slide substrates were affixed was placed directly behind the polarizer in the light path. Rotation of the sample holder resulted in illumination of the same area of the sample both in the parallel (0°, shear direction parallel to the electric field vector of the incident radiation) and perpendicular (90°, shear direction perpendicular to the electric field vector of the incident radiation) configurations. The UV-vis spectrum of the glass substrate was stored as background, and all spectra of the solid films were internally background subtracted. Photomicrographs were taken using a Nikon 600Epol microscope with strain-free objectives and a Nikon N70 single lens reflex camera mounted on a trinocular head. Infrared spectra were recorded on a Perkin-Elmer Spectrum 2000 FT-IR spectrometer. Spectra of powdered samples were obtained in the form of KBr pellets prepared with dried KBr using a minipress from SpectraTech, Inc. Aligned solid films for polarized infrared spectroscopy were prepared by shearing liquidcrystalline solutions of 1 between two 25 mm diameter KRS-5 (thallium bromide/iodide crystal) windows (Aldrich, Milwaukee, WI) followed by evaporation of solvent. Polarized IR transmission spectra were collected by placing a KRS-5 wire grid polarizer (2-35 µm, International Crystal Lab., Garfield, NJ) in the incident light path in the sample chamber, with the resulting polarized IR radiation incident at a normal angle to the coated KRS-5 substrate. To confirm alignment of the samples on the KRS-5 substrates, we examined their optical textures at high magnification with a polarizing microscope (Nikon 600Epol) after
Figure 3. Series of offset visible spectra for aqueous solutions of 1. obtaining the IR spectra. The optical textures observed were similar to those observed for the analogous thin films on glass. Small-angle powder X-ray diffraction studies were performed using an Inel CPS 120 powder diffraction system with monochromatic Cu KR radiation. The sheared film of 1 was coated onto a cleaned glass microscope slide and analyzed with the X-rays, while the liquid crystalline solution was analyzed using capillary tubing.
Results and Discussion Self-Organization of 1 in Aqueous Solutions. UVVisible Spectroscopy. To understand the self-organization of the dye molecules in water, we examined dilute solutions of 1 with UV-vis spectroscopy, a technique useful for examining aggregation in solution.15 Upon variation of the concentration over a wide range, significant changes in the visible spectrum due to aggregation were observed (Figure 3). At very low concentrations (∼1 × 10-7 to 8 × 10-8 M), 1 has a λmax at 535 nm with two smaller peaks at 500 and 470 nm. The two peaks at 500 and 470 nm gradually increase in relative intensity with increasing concentration. At higher concentrations (∼1 × 10-2 M), the spectrum shows that the peaks at 500 and 470 nm merge, resulting in a very broad peak with a λmax at 488 nm. The dramatic changes in the electronic transitions observed in the visible spectra with increasing concentration are indicative of intermolecular orbital interactions between the polyaromatic rings16 and, thus, aggregation of the molecules. The peak at 535 nm is most likely due to absorption of the monomeric dye molecules.17 Upon increasing the concentration, the peak at 500 nm and the (15) (a) West, W.; Pearce, S. J. Phys. Chem. 1965, 69, 1894-1903. (b) Tiddy, G. J. T.; Mateer, D. L.; Ormerod, A. P.; Harrison, W. J.; Edwards, D. J. Langmuir 1995, 11, 390-393. (c) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. 1996, 100, 2310-2321. (16) Kazmaier, P. M.; Hoffmann, R. J. Am. Chem. Soc. 1994, 116, 9684-9691. (17) Ford, W. E. J. Photochem. 1987, 37, 189-204.
Controlling Molecular Orientation
Figure 4. Optical photomicrograph of a solution of 1 in water (200×, crossed polarizers). (Top) The peripheral evaporation experiment shows the anisotropic liquid (N phase) developing out of the isotropic (black) liquid (1.7 mM) as the concentration gradient increases from top to bottom. (Bottom) A concentrated solution (0.3 M) of 1 in water.
shoulder at 470 nm are amplified as the amounts of dimer and higher aggregates increase. Since the absorption peak is blue-shifted as the concentration increases, the aggregates are referred to as H-aggregates and are conventionally thought of as untilted or slightly tilted (slippage angle larger than about 32°) ladder-like stacks.18 Polarized Optical Microscopy. At a concentration of 1.7 × 10-3 M, solutions of 1 were still isotropic (I) as observed by polarized microscopy (black area of Figure 4). When this isotropic solution under the cover glass was concentrated (by allowing water to evaporate from the edge of the sample), birefringent droplets (Figure 4, top) isolated from each other in the isotropic bulk were observed, indicating the formation of a liquid-crystalline phase. The emergence of the liquid-crystalline phase from the isotropic solution appeared at about 0.1 M in concentration. As the concentration increased, these droplets coalesced to form a unified bulk liquid-crystalline phase. Although the studies by UV-vis spectroscopy indicate that dimerization of 1 begins at concentrations lower than 10-7 M, the aggregation buildup occurs over 6 orders of magnitude in (18) Czikklely, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207-210.
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concentration before the aggregates are large enough to have long-range orientation order and show birefringence when observed between crossed polarizers in a microscope (typically at 200× to 600×). The photomicrograph (Figure 4, bottom) taken of a more concentrated solution of 1 (0.3 M) indicates that the sample is completely liquid crystalline with observed textures characteristic of a chromonic N phase (a nematic phase).7a In this type of phase, the mesogens typically stack to form columns (not necessarily simple one-molecule-wide columns), but there is no positional order among the columns. Small-Angle X-ray Diffraction (XRD). A liquid-crystalline sample of 1 (∼8 wt %) in aqueous solution was analyzed by small-angle X-ray diffractometry. A broad Bragg reflection peak was observed in the wide-angle region corresponding to a d spacing of about 3-4 Å; this is presumably due to the spacing between the perylene rings within a column. The broadness of the peak may be attributed to disordered π-stacking and/or the presence of water.19 In the small-angle region, no diffraction peak was observed for this sample; presumably, the spacing is too large to be detected with the instrument used. When more concentrated samples of 16 and 25 wt % were studied, diffraction peaks at 2.4° and 2.8° (2θ) were observed, respectively, suggesting that the intercolumn spacing decreases with more concentrated samples. The spacing for the 25 wt % sample is calculated to be approximately 31 Å. Anisotropic Thin Solid Films. Polarized Optical Microscopy. We prepared anisotropically oriented films of 1 by simultaneously depositing and aligning the aqueous solution (in the chromonic N phase) onto glass substrates and then allowing the water to evaporate under ambient conditions. Upon examination of these thin solid films by polarized microscopy, it became evident that these films acted as good polarizing materials. When the polarization axis of the incident light was parallel to the shearing direction, the films were colorless and transparent (Figure 5, top). In contrast, only red light was transmitted (blue and green light were absorbed) when the polarization axis of the incident light was orthogonal to the shearing direction (Figure 5, bottom). When a liquid-crystalline solution of 1 was allowed to dry on a glass slide without alignment with a shearing force, no bulk anisotropic orientation or light polarization on a macroscopic scale was observed. Polarized UV-Visible Spectroscopy. The optical properties of these films were further studied using polarized visible spectroscopy. Figure 6 shows the polarized visible spectra in transmission mode of the sheared film, with the incident radiation normal to the film surface. The film exhibited intense absorption of blue and green light when the polarization axis of the incident light was perpendicular to the shearing direction, while only weak absorption occurred when the polarization axis of the incident light was parallel to the shearing direction. This indicates that the majority of the molecules on the glass substrate are oriented with their long axes orthogonal to the shearing direction, since the electronic transition moment for this transition of 1 is along the long axis of the molecular plane.20 The dichroic ratio (Rd), a ratio of the absorbances (A(90°) and A(0°)) associated with orthogonal directions in the plane of the film, is a good measure of the degree of (19) Mariani, P.; Mazabard, C.; Garbesi, A.; Spada, G. P. J. Am. Chem. Soc. 1989, 111, 6369-6373. (20) Hasegawa, M.; Matano, T.; Shindo, Y.; Sugimura, T. Macromolecules 1996, 29, 7897-7909.
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Figure 7. The dichroic ratio (Rd) vs wavelength plot for the film with optical properties shown in Figure 6. (Rd) ) A(90°)/ A(0°).
Figure 5. Optical photomicrograph of an air-dried film prepared from an 8 wt % solution of 1 with the shearing axis in the vertical direction: (top) polarization axis of incident light parallel to the shearing direction, and (bottom) polarization axis of incident light perpendicular to the shearing direction (600×).
Figure 6. Optical properties of a solid film of 1 on glass prepared from a 9 wt % aqueous solution. T(90°) is the transmittance when the aligned film’s shearing axis is perpendicular to the polarization of the incident light. T(0°) is the transmittance when it is parallel. The mean transmittance (T0) is calculated as [T(0°) + T(90°)]/2 and is a measure of the transmittance of unpolarized incident light. The degree of polarization (V) is calculated as 100% × [T(0°) - T(90°)]/[T(0°) + T(90°)].21
anisotropic orientation of the molecules in the film since it is theoretically independent of film thickness.21 The variation of the dichroic ratio with wavelength is shown in Figure 7. Although the coating conditions have not been optimized,22 the maximum values of Rd for these films routinely exceed 25 (and in some cases the value exceeds 30), which is considered an excellent value for a polarizing material.23 The high dichroic ratio indicates that the molecules in these films are highly anisotropically oriented. (21) West, C. D.; Jones, R. C. J. Opt. Soc. Am. 1951, 41, 976-986. (22) A detailed investigation of the effect of concentration and coating conditions on the optical properties of the films is currently underway.
Polarized Transmission Infrared (IR) Spectroscopy.24 Infrared spectroscopy is extremely useful for determining solid-state molecular orientation, especially when the incident light is polarized. The technique was initially deployed on molecular crystals to determine the assignments of the vibrational modes of the crystalline compounds.25 Once the in-plane and out-of-plane modes of the molecule have been assigned, differences in the relative intensities of these peaks in the polarized IR spectrum can help determine molecular orientation in the sample.26 Polarized transmission IR spectroscopy was used to further elucidate the orientation of 1 in the sheared films. The complete peak assignments for the vibrational modes of bis(alkylamino)perylene-3,4,9,10-tetracarboxylic diimides have been previously reported in the literature.27 The principal in-plane bands at 1697 and 1659 cm-1 correspond to the OdCN stretches while the in-plane bands at 1594 and 1577 cm-1 correspond to the CdC aromatic stretches. The principal out-of-plane bands are at 745 and 809 cm-1 and have been assigned as C-H perylene wagging modes. All of these bands were observed in the reference spectrum of unoriented 1 in KBr pellets (Figure 8a). Changes in the relative intensities of the inplane modes or the out-of-plane modes in the sheared films with respect to those of 1 in KBr yield information concerning the orientation of the molecules in the films. The sheared samples on the KRS-5 windows produced spectra that exhibit dramatic differences in relative peak intensities upon rotation of the sample with respect to the axis of polarized IR radiation. When the shearing direction of the film and the electric vector of IR polarization were parallel, the 809 and 745 cm-1 peaks were greatly enhanced (Figure 8b) in relative intensity with respect to the normally dominant in-plane stretches at 1697 and 1659 cm-1. Upon rotation of the sheared sample 90°, so that the shearing direction was perpendicular to the electronic vector of the incident IR radiation, the opposite effect was seen as all of the in-plane imide and aromatic stretches (1697, 1659, 1594, and 1577 cm-1) increased in (23) Shurcliff, W. A. Polarized Light; Harvard University Press: Cambridge, MA, 1962. (24) Thulstrup, E. W.; Michl, J. Spectrochim. Acta 1988, 44A, 767782. (25) Ambrosino, F.; Califano, S. Spectrochim. Acta 1965, 21, 14011409. (26) Chollet, P.-A.; Messier, J.; Rosilio, C. J. Chem. Phys. 1976, 64, 1042-1050. (27) (a) Maiti, A. K.; Aroca, R.; Nagao, Y. J. Raman Spectrosc. 1993, 24, 351-356. (b) Rodriguez-Llorente, S.; Aroca, R.; Duff, J. Spectrochim. Acta 1999, 55A, 969-978.
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Figure 8. (a) FTIR spectrum of randomly oriented 1 in a KBr pellet. (b) Polarized FTIR spectrum of an oriented film of 1 on a KRS-5 substrate, with the incident light polarized parallel to the shearing direction. (c) The same film and same incident polarization as (b) with the sample rotated by 90°.
Figure 9. Schematic diagram of a molecule of 1 on the glass substrate in the sheared film with defining axes labled. The planes of the relevant IR dynamic dipoles of the molecule are also represented.
relative intensity while the peaks at 809 and 745 cm-1 almost disappeared (Figure 8c). The out-of-plane C-H perylene wagging resonances at 745 and 809 cm-1 are particularly useful in assigning the orientation of the molecular plane of 1 with respect to the substrate plane.28 With the substrate in the XY plane and the shearing direction and the polarized IR radiation parallel to the X axis, the C-H perylene wagging resonance will be strongest when the molecular plane is in the YZ plane (Figure 9). This molecular orientation causes the dynamic dipole vector (µ) of the C-H perylene wagging mode to lie along the X axis (the axis of polarized IR irradiation). If the average molecular plane was in the XZ plane or in the XY plane parallel to the substrate surface, the dipole vector of the C-H wagging mode would be perpendicular to the axis of polarized IR radiation, and the C-H wagging resonance would not be observed. The results shown in spectra b and c of Figure 8 provide strong (28) Rodriguez-Llorente, S.; Aroca, R.; Duff, J.; de Saja, J. A. Thin Solid Films 1998, 317, 129-132.
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evidence that the molecular plane of 1 in these oriented films is aligned with the YZ plane. The dramatic change in the intensities of the in-plane stretching resonances (1697, 1659, 1594, and 1577 cm-1) upon rotating the sheared sample 90° with respect to the axis of IR radiation also supports the conclusion that the average molecular plane of 1 is aligned with the YZ plane. When the shearing direction is parallel to the polarized IR radiation, the peaks for the in-plane resonances are very weak (Figure 8b). This is consistent with alignment of the molecular plane with the YZ plane because the molecular plane of 1 and hence the in-plane stretch dipoles are orthogonal to the IR polarization axis. When the shearing direction is perpendicular to the polarized IR radiation, the peaks for in-plane resonances are strong since the dipoles and the IR radiation are parallel (Figure 8c). In the nematic phase, the director of the liquid crystal aligns with the long axis of the columns formed by the H-aggregate-type stacking of the molecules. Mechanical shearing aligns the directors (and thus the long axis of the columns) along the shearing direction,8b,i resulting in the edge-on orientation of the molecules. It is not conclusive from the results of the polarized transmission IR studies whether the long axis of 1 lies along the Y or Z axis. However, the intense absorption of visible light polarized along the Y axis of the sheared film (discussed in the previous section) suggests that a significant number of molecules have a component of their electronic transition moment (which is parallel to the long axis of the molecule) projected along the Y direction. It is interesting to compare the average molecular orientation in these liquid-crystal films to that in films prepared by vapor deposition of a perylene diimide analogue onto a silver-coated glass substrate, as reported by Rodriguez-Llorente, et al.28 By transmission and reflection-absorption IR spectroscopies, the average molecular plane of the perylene analogue in the vapor deposited films was determined to be face-on (parallel to) the substrate; however, no preferred orientation of the long axis of the molecules was reported. Small-Angle X-ray Diffraction (XRD). X-ray diffraction data were collected on a sheared solid film of 1 and were compared to the XRD data of the nematic liquid-crystalline phase from which the films were prepared. The peak around 3-4 Å remained broad in the film samples, presumably still due to disordered π-stacking and/or the presence of residual water in the air-dried films.29 In the small-angle region, several intense Bragg reflections were observed from the sheared films indicating long-range periodicity of molecules in the films (in contrast to the nematic liquid crystalline phase used to prepare the films). The first and second Bragg reflections were typically the most intense and match well with lattice spacings of d and d/x3 (where d varied from 54 to 68 Å depending on the individual film).30 This pattern might be consistent with a hexagonal arrangement of the liquid-crystalline columns in the sheared films; however, other characteristic (29) The presence of water was also indicated by the presence of a broad IR absorption peak around 3400 cm-1 in these films (not shown in Figure 8). The water content of the air-dried films was estimated, from the weight loss of the films after heating at about 100 °C under vacuum overnight, to be about 15 wt % or less. For other examples of chromonic mesogens showing a broad hump around 3.5 Å in d spacing in the X-ray diffraction patterns of their “quasi-crystalline” solids, see: Sadler, D. E.; Shannon, M. D.; Tollin, P.; Young, D. W.; Edmondson, M.; Rainsford, P. Liq. Cryst. 1986, 1, 509-520. (30) The presence of residual water molecules and irregular widths of the columns (since the aggregates are not necessarily simple onemolecule-wide columns) presumably results in the variation in the main d spacing that was observed from sample to sample.
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hexagonal packing diffraction peaks (such as d/x4, d/x7, d/x9) are absent or very weak. These intense Bragg peaks do indicate an increased order in the sheared solid film over the liquid nematic phase. Conclusion We have shown that a liquid-crystalline phase exists in aqueous solutions of cationic perylene diimide 1 and have concluded that it belongs to a class of lyotropic mesogens referred to as chromonic liquid crystals. Utilizing polarized UV-vis and IR spectroscopies, we determined the average molecular orientation in thin solid films of 1 produced by inducing macroscopic orientation from the self-organized liquid-crystalline solutions. The molecules in these films are highly anisotropically oriented with the average molecular plane edge-on (perpendicular to) the substrate surface and with the long molecular axis oriented orthogonal to the shearing direction. Our work shows that a different molecular orientation and anisotropic order in
Iverson et al.
solid films can be obtained by exploiting the liquidcrystalline properties of the perylene diimides as compared to vapor deposition. X-ray diffraction data suggest that the alignment of the nematic liquid-crystalline solutions yields materials with a higher degree of molecular order than the nematic mesophase. This simple strategy is capable of generating films that are good polarizers with dichroic ratios as high as 30, rivaling some commercial sheet polarizers. Acknowledgment. S.-W.T.-C. is grateful for the CAREER Award granted by NSF (DMR-9876027). We thank Professor Douglas Gin (Department of Chemistry, University of California, Berkeley) for granting use of the X-ray diffractometer and for helpful suggestions on interpreting the data. We appreciate the comments and suggestions of the reviewers. LA011499T
